Should I build pyramids or overviews?

This is a very common question that many users ask. First let’s take a look at what each of these, and when they are needed.

Pyramids
Pyramids are reduced resolution datasets. They let you perform dynamic mosaicking and on-the-fly processing quickly, even at smaller scales with little effect on performance. Pyramids are built for primary (source) rasters that are on disk. `Viewing and processing these raw rasters on-the-fly can be time-consuming at small scales, therefore we build pyramids for this reason.

Overviews
Overviews are similar to pyramids in the sense that they are reduced resolution images created to optimize your mosaic dataset for performance and increase display speeds. However, the biggest distinguishing factor here is that the overviews are not created per raster. Each overview is derived by mosaicking multiple primary rasters. Since each overview was derived from multiple rasters, it takes a static view of the mosaic dataset at the time you generate overviews. Creating overviews will mean that the dynamic mosaicking capabilities are not longer available.

When are pyramids per mosaic dataset item needed
You should consider building pyramids when you are building more complex mosaic datasets, especially where you will be taking advantage of the mosaic methods and on-the-fly processing, then it will be advantageous to build pyramids on the source rasters and to build overviews only where they are needed.

When are overviews needed in a mosaic dataset
You should consider building overviews when you are working with: tiled images, preprocessed images, butt-joined (non-overlapping) imagery, basically imagery that wont be really affected by changing mosaic methods. But one thing to keep in mind, overviews do not retain the primary rasters metadata.

6 Comments

kmayallsays:

When you are using a mosaic dataset/overviews with tiled preprocessed imagery, should you still build pyramids for when you are working with the mosaic at a large map scale and therefore looking at the source tiles?

Kmayall asked if pyramids were needed if they see the source imagery already. In order to see the source imagery (when pyramids or overviews have been built), that would mean you need to zoom into the pixel resolution.

I have created pyramids for a set of rasters in a mosaic. The odd thing is we can’t see any pyramids oustide a certain scale (377,000), but didn’t set it up that way. I have rerun and get the same thing. Oddly I have done same techniques on other mosaics, and can see pyramids at all scales.

I have found nothing in the help or blogs describing this. Anyone else get this? Any fixes?

Pyramids can be created for a mosaic dataset, but the mosaic dataset has several things that affect the visibility of the images. The easiest fix would be to simply create overviews for the mosaic dataset. But that is obviously building overviews on the mosaic dataset as a whole.
The real fix to get your raster images (and pyramids) to show up at all scales is to edit the visibility of the mosaic dataset. However this requires a bit of work with the Calculate Cell Size Ranges tool. This tool tell the software at which pixel resolutions to display or not display the data. If the default does not work for your data, then try increasing your Maximum Cell Size Factor to 1000 or greater.